Demonstrators stand with a road sign during protests near Ferguson, Missouri.

Reuters: Lucas Jackson

Police came under "heavy gunfire" and 31 people were arrested in the latest racially-charged protests sparked by the fatal shooting of an unarmed black teenager by a white policeman in Ferguson, Missouri.

Highway Patrol Captain Ron Johnson said two people were also hit by gunfire from demonstrators in the St Louis suburb.

"Our officers came under heavy gunfire," in one area, he said, adding that riot police had confiscated two guns from protesters and what looked like a petrol bomb.

Demonstrations, mostly peaceful but with spasms of violence by smaller groups, have flared since Michael Brown, 18, was shot dead during an incident with a policeman in a patrol car while walking down a residential street in Ferguson on August 9.

The clashes between riot police and protesters on Monday night occurred after hours of demonstrations that were mostly peaceful, Reuters witnesses said.

Police had closed a roadway to traffic to provide a path for marches but said a smaller group within the larger crowd hurled bottles, rocks and petrol bombs at officers standing near armoured vehicles.

Police responded by firing gas-filled canisters and a noise cannon to try to disperse the throng.

"These criminal acts came from a tiny minority of lawbreakers. But anyone who has been at these protests understands that there is a dangerous dynamic in the night," Captain Johnson said.

"It allows a small number of violent agitators to hide in the crowd and then attempt to create chaos. The catalyst can be bottles thrown, Molotov cocktails and of course, shots fired."

Some demonstrators, including a church minister using a blow horn, urged crowds to calm down.

"With these additional resources in place, the Missouri State Highway Patrol and local law enforcement will continue to respond appropriately to incidents of lawlessness and violence and protect the civil rights of all peaceful citizens to make their voices heard," he said in a statement.

"We will not use a curfew tonight."

The Federal Aviation Administration said it had renewed a ban on low-flying civilian aircraft over Ferguson to help law enforcement authorities do their job.

Obama says use of National Guard should be limited

Mr Brown's death prompted often violent protests in Ferguson, which has an African-American majority but a mainly white police force.

The National Guard deployment was the latest step by authorities to end the looting and burning of stores that have punctuated protests and stirred questions about race relations in the United States.

National Guard troops could be seen walking on the fringes of the gathering, keeping a distance from protesters.

US president Barack Obama said the use of the National Guard should be limited and urged healing instead of violence, adding that attorney-general Eric Holder would travel to Ferguson on Wednesday.

"While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns, and even attacking the police, only serves to raise tensions and stir chaos," Mr Obama told a news conference on Monday.